The invention relates to a drive mechanism which is intended to be used particularly for the closing unit or the injection unit or the ejector of a plastics injection molding machine having an output element (54) that can be moved in a straight line and an electric-motor drive unit (50) having an electric motor (51), and having at least a first piston-cylinder unit (12), which has a first cylinder housing (13) and first hydraulic piston (14) which, with a first active surface (18, 19), bounds a first cylinder chamber (16, 17) filled with pressurized liquid and lies in the force chain between the output element (54) of the electric-motor drive unit (50) and the movable component (10, 66).
Inside the closing unit of a plastics injection molding machine, the drive mechanism moves the movable mold clamping plate of the machine. A drive mechanism of this type has to satisfy two important different requirements. Firstly, it is to move the mold clamping plate as quickly as possible in order to close and to open the mold, in order that the cycle time for the production of a molding can be kept short. Secondly, it must be able to hold the mold clamping plate and therefore the entire mold against the high injection pressure with great force. Firstly, therefore, actuating movements have to be carried out at high speed, secondly high forces have to be exerted without significant movement. Apart from on the closing unit, such requirements can also be placed on the ejectors or the injection unit of a plastics injection molding machine. For example, when plastic is injected into the mold, the plasticizing screw is moved at relatively high speed in the direction of the mold until the mold is completely filled with plastic. If the plastic melt located in the mold is then subjected to post-injection pressure, as it is known, then the drive has to apply a high force without substantial movement of the plasticizer screw.
U.S. Pat. No. 4,030,299 discloses a purely hydraulic drive for the movable mold clamping plate of a plastics injection molding machine, which also contains a hydraulic device for force multiplication (a hydraulic force multiplier). The latter has a movable hydraulic piston with a small active surface, a further movable hydraulic piston with a large active surface and a cylinder which, together with the hydraulic pistons, encloses a cylinder chamber filled with a pressurized liquid. The cylinder is arranged in a fixed location on the frame of the injection molding machine. The drive additionally includes hydraulic cylinders which are fed by a hydraulic pump and which move the movable mold clamping plate in order to close and open the mold. When the mold is open, the volume of the cylinder chamber of the hydraulic force multiplier is a minimum. If, then, the movable mold clamping plate is moved by the hydraulic cylinders with the effect of closing the mold, then the large hydraulic piston of the hydraulic force multiplier is carried along as well, the volume of the cylinder chamber of the hydraulic force multiplier being increased and pressure media flowing into the cylinder chamber from a container via a suction follower valve. When the movable mold clamping plate has reached its front end position, the suction follower valve closes. Further pressure medium is supplied to the cylinder chamber of the hydraulic force multiplier by a second hydraulic pump via a directional control valve, as a result of which the pressure in the cylinder chamber rises. When a specific pressure has been reached, the directional control valve is changed over and a hydraulic cylinder that actuates the small hydraulic piston of the hydraulic force multiplier is supplied with pressure medium by the second hydraulic pump. The small hydraulic piston moves into the cylinder chamber, in which a high pressure is produced, which effects a high closing force via the large active surface of the large hydraulic piston. In the drive mechanism according to U.S. Pat. No. 4,030,299, there are therefore various hydraulic drive components for the actuating movement of the movable mold clamping plate and for the exertion of a high force.
A drive mechanism of the introductory-mentioned type is disclosed by DE 41 11 594 A1. In the case of this drive mechanism, a piston-cylinder unit with a hydraulic piston with a large active surface is permanently connected to the movable mold clamping plate. The unit comprising the movable mold clamping plate and piston-cylinder unit can be moved by an electric motor via gear mechanism which comprises a reciprocating spindle and a spindle nut, in order to close the mold quickly and to open it quickly. The high closing force is applied by applying pressure to the piston-cylinder unit that can be moved with the mold clamping plate. In this case, the entire-reaction force is dissipated to the machine frame via the spindle and the spindle nut. Apart from the components of the electric drive, the plastics injection molding machine according to DE 41 11 594 A1 is also equipped with a complete hydraulic system including oil container, pump, valves and piston-cylinder unit.
The invention is based on the object of developing a drive mechanism which has the features of the introductory-mentioned type in such a way that, with little expenditure, on the one hand a rapid actuating movement is possible and on the other hand a high effective force can also be achieved.